This invention relates to a precursor of and a method to prepare high-Ni “NMC” cathode powdery material at large scale and at low cost. By “NMC” we refer to lithium-nickel-manganese-cobalt-oxide. The high-Ni NMC powder can be used as a cathode active material in Li-ion rechargeable batteries. Batteries containing the cathode materials of the invention yield better performances, such as a higher cycle stability and a low content of soluble base.
Currently rechargeable Li-ion batteries start penetrating the “large” rechargeable battery market. Here, “large batteries” refers to applications such as automotive batteries, as well as stationary power stations. These large stationary or automotive batteries are much larger than previous dominating batteries for portable applications, like cylindrical cells for notebooks or polymer cells for smartphones. Therefore there are fundamentally different requirements for the “large battery” cathode materials, not only performance-wise, but also from the point of resource scarcity. Previously the majority of rechargeable lithium batteries used LiCoO2 (LCO) as cathode material. LiCoO2 is not sustainable for large batteries due to limited cobalt resources—as already today about 30% of the earth's available cobalt is used for batteries, according to the Cobalt Development Institute. The situation is less critical for the so-called NMC cathode materials. Examples are “442” and “532” cathode materials; 442 generally referring to Li1+xM1−xO2 with x=0.05 and M=Ni0.4Mn0.4Co0.2; and 532 generally referring to LiMO2 with M=Ni0.5Mn0.3Co0.2. The NMC cathode materials contain less cobalt since it is replaced by nickel and manganese. Since nickel and manganese are cheaper than cobalt and relatively more abundant, NMC potentially replaces LiCoO2 in large batteries. Other candidates as olivines (LiFePO4) are less competitive because of the much lower energy density compared to NMC.
A NMC cathode material can roughly be understood as a solid state solution of LiCoO2, LiNi0.5Mn0.5O2 and LiNiO2. In LiNi0.5Mn0.5O2 Ni is divalent, in LiNiO2 Ni is trivalent. At 4.3 V the nominal capacity for LiCoO2 and LiNi0.5Mn0.5O2 is about 160 mAh/g, against 220 mAh/g for LiNiO2. The reversible capacity of any NMC compound can be roughly estimated from these given capacities. For example NMC 622 can be understood as 0.2 LiCoO2+0.4 LiNi0.5Mn0.5O2+0.4 LiNiO2. Thus the expected capacity equals 0.2×160+0.4×160+0.4×220=184 mAh/g. The capacity increases with “Ni excess” where “Ni excess” is the fraction of 3-valent Ni; in NMC622 the Ni excess is 0.4 (if we assume lithium stoichiometry with Li:(Ni+Mn+Co)=1.0). Obviously the capacity increases with Ni excess, so that at the same voltage, Ni-excess NMC possesses a higher energy density than LCO, which means less weight or volume of cathode material is required for a certain energy demand when using Ni-excess NMC instead of LCO. Additionally due to the lower price of nickel and manganese—compared to cobalt—the cost of cathode per unit of delivered energy is much reduced. Thus, the higher energy density and lower cost of Ni-excess NMC—by contrast to LCO—is more preferred in the “large battery” market.
A simple and cheap manufacturing process of NMC cathode material is required for a large-scale application. Such a typical process—which we call direct sintering—is the firing of a blend of a mixed metal precursor (for example M(OH)2 precursor) and a lithium precursor (for example Li2CO3) in trays, in a continuous manner. Trays with blends are continuously fed into a furnace, and during the movement through the furnace the reaction towards the final sintered LiMO2 proceeds. The firing cost depends strongly on the thru-put of the firing process. The faster the trays move across the furnace (referred to as the “firing time”) and the more blend the trays carry (referred to as the “tray load”) the higher the thru-put of the furnace is. A furnace has a high investment cost, therefore, if the thru-put is small, the furnace depreciation significantly contributes to the total process cost. In order to achieve a cheap product, a high thru-put is thus desired.
As the capacity of NMC material increases with Ni excess, “Ni-excess” NMC cathode materials, like NMC 532 and NMC 622, possess a higher capacity in batteries than with less Ni, as for example NMC 111 (being LiMO2 with M=Ni1/3Mn1/3Co1/3, Ni excess=0). However, the production becomes more and more difficult with increasing Ni content. As an example—very high Ni-excess cathode materials like NCA (which is LiNi0.8Co0.15Al0.5O2) cannot be prepared in air or using Li2CO3 as Li precursor. Because of the low thermodynamic stability of Li in high-Ni material, the preparation occurs in CO2 free oxidizing gas (typically oxygen) and as lithium precursor LiOH is used instead of the cheaper Li2CO3. Contrary to this, the low Ni NMC111 can easily be prepared in normal air and using a Li2CO3 precursor. As Ni increases NMC tends to have a low air stability, and it is more difficult to obtain a cathode with low content of soluble base. The concept of “soluble base” is more explicitly discussed in U.S. Pat. No. 7,648,693.
The preparation of NMC 532 (Ni excess=0.2) is more difficult than NMC 111, but NMC 532 can be processed at large scale through the cheap and simple “direct sintering” solid state reaction under air. The lithium source is preferably chosen as Li2CO3, as in the production of NMC 111, due to its low price. This detailed preparation procedure of direct sintering will be discussed in the description below. The manufacturing cost of NMC 532 cathode material is relatively higher than NMC 111 but much cheaper than for NCA that has a Ni excess of 0.8.
The energy density of NMC 532 is much higher than NMC 111, thus, NMC 532 is very competitive to take the place of LCO in a cheap production process at large scale.
Another promising Ni-excess NMC is NMC 622, whose Ni excess is 0.4, being much higher than the Ni excess in NMC 532, so that the capacity of NMC 622 is still higher than that of NMC 532, but at the same time, the production is more difficult than NMC 532, and definitely harder than NMC111. Even if it might still be feasible to produce NMC 622 by direct sintering, as in the case of NMC 532 and NMC 111, however, it is difficult to prepare NMC 622 at large scale at low cost in an efficient way. The problems of large production mainly originate from the high soluble base content in the final NMC product. The soluble base refers to the surface impurities like Li2CO3 and LiOH, and in this case the Li2CO3 impurity is of most concern. As discussed in U.S. Pat. No. 7,648,693, these bases may come from unreacted reagents of lithium sources, usually Li2CO3 or LiOH.H2O, where LiOH.H2O normally contains 1 wt % Li2CO3 impurity. These bases may also come from the mixed transition metal hydroxides that are used as transition metal source in the production. A mixed transition metal hydroxide is usually obtained by co-precipitation of transition metal sulfates and an industrial grade base such as NaOH. The base contains a CO32− impurity in the form of Na2CO3. In the case of high Ni-excess NMC, like NMC 622, after sintering at high temperature, the carbonate compounds remain on the surface of the final product. The soluble base content can be measured by a technique called pH titration, as discussed in U.S. Pat. No. 7,648,693.
The presence of soluble base content in the final NMC material could cause a serious gas generation in full cells, which is usually called “bulging” in full cell tests. Serious gas generation or bulging issues will result in a bad cycling life of the battery together with safety concerns. Therefore, in order to use high Ni-excess NMC material for large battery applications, an effective and cheap processing method is necessary that avoids such high soluble base content. Additionally it is observed that the deterioration of cyclability in NMC material is related to the above-mentioned presence of Li2CO3.
A process to prepare NMC 622 with low Li2CO3 soluble base—as is disclosed in US2015-010824—runs as follows: LiOH.H2O with low Li2CO3 impurity as Li source, is blended with mixed transition metal hydroxide at target composition, and sintered at high temperature under an air atmosphere. In this process, the base content of such high Ni-excess NMC final product (like NMC 622) is much reduced, but the manufacturing cost is relatively high due to the higher price of pure LiOH.H2O compared to a Li2CO3 precursor. This conflicts with the low cost benefit of substituting LCO by NMC material, where, as said before, a cheap and simple production process is essential to replace LCO.
U.S. Pat. No. 7,648,693 proposes a “split” method, where the direct sintering is conducted in two steps: a first lithiation at relatively low temperature, like 700° C., and a second step of sintering at a higher temperature. In this patent, a large-scale preparation of LiMO2 with M=Ni4/15(Mn1/2Ni1/2)8/15Co0.2 is achieved with a final product that is almost free of soluble base. The cycling stability of that NMC material is also improved. The “split” method is thus a potential way to prepare NMC 622 free of soluble base and at low cost. However, it has been found that this “split” method is not usable for the large scale production of NMC 622, with lithium carbonate as Li-precursor, as in U.S. Pat. No. 7,648,693 excessive amounts of preheated air have to be pumped through the reactor. Practically this processing method is limited for lower Ni-excess NMC, such as NMC 532.
Therefore, in order to replace LCO by high Ni-excess NMC—like NMC 622—for the “large battery” market, it is the aim of the present invention to provide a cheap and efficient manufacturing process, where the high Ni-excess NMC can be produced at low cost, and without resulting in a too high soluble base content.